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Eli Lilly and Co. has three Alzheimer's drug candidates in clinical trials, each attacking the disease in a different way.

The Indianapolis drug ­maker's progress has been halting: Two of the drugs stumbled over safety or efficacy issues during tests, and the third is in the earliest stage of human testing, still years away from getting onto the market.

The candidate that excites Wall Street the most for its sales potential is solanezumab, an intravenous treatment that targets the buildup of brain plaque known as beta amyloid, thought to be a basis of Alzheimer's. ...